Googling for ways to finish "(just) grew like..." ("gangbusters" and
"wildfire" are popular choices), I came across a nice mondegreen that
appeared in the Arizona Daily Star:

        http://www.dailystar.com/dailystar/relatedarticles/4644.php
        The Amado Territory Ranch is anchored by an 11-room
        bed and breakfast inn, Amado Territory Inn, that has
        spectacular views and serene natural surroundings. The
        rest of the land was quickly leased by other vendors.
        "It just grew like top seed," Art Gould said.

Clearly the original expression, "just grew like Topsy" (referring to
Topsy in _Uncle Tom's Cabin_, who "jes grew"), was unknown to Mr. Gould,
or the reporter, or both.  But who knows, this could be a folk etymology
in progress -- as the original referent of "Topsy" becomes less and less
familiar, the term could get reanalyzed as "top seed", which sounds like
something that might grow quickly.